Effect of aluminum speciation and pH on in-line coagulation/diatomite microfiltration process: Correlations between aggregate characteristics and membrane fouling

Impacts of aluminum speciation and water pH on coagulation/diatomite microfiltration (DMM) process were investigated in this study. Three commonly used Al-based coagulants, aluminum sulfate (AS), poly-aluminum chloride (PACl), and Al13 ([Al13O4(OH)24(H2O)12]7 +), were employed to explore the role of Al speciation. Floc characteristics, including particle size and fractal structure under different coagulants and water pH levels, were investigated using a laser diffraction particle sizing device. The results indicated that in the dosage range of 1.0-11.0 mg/L, coagulation/DMM could effectively remove particles (97-100%) and DOC (40-80%), and as pH increased from 3.0 to 10.0, its purification efficiency was higher and more stable than separate coagulation. AS contributed to large and loosely structured flocs and thus alleviated the flux declines most effectively, leading to a flux reduction of about 50%, while the reductions for PACl and Al13 were 71% and 73%, respectively. Neutral pH was found most conducive to the hybrid process and alkaline pH (9.0) led to relatively larger flux than strong acidic pH (3.0), especially for AS coagulation system. As pH changed, Al13 led to the least variation in flux declines while both the removal efficiency and filtration flux fluctuated most obviously for AS coagulation system.